1. Field of the Invention
The present invention relates generally to an electronic device that can improve packaging density. More specifically, the invention relates to an electronic device that can improve the efficiency of an electrical connection between circuit boards and the effect of heat radiation, and prevent adverse effects from heat radiation, in a multi-board construction of the circuit boards. Furthermore, the invention relates to an electronic device and a lead and a terminal adapted to the electronic device for facilitating a connection of the boards and a lid member.
2. Description of the Related Art
In the prior art, there are known electronic devices in which two or more circuit boards having installed semiconductor integrated circuits are packed in a single package. (For example, Japanese Unexamined Patent Publication (Kokai) No. 1-147850).
For such devices, various improvements have been made for facilitating the wiring between two or more circuit boards. Also, for external signalling, lead pins have been employed.
Conventionally, two or more circuit boards forming a multi-board structure have been bonded within the package.
In such a construction of the electronic device, the heat generated in the semiconductor circuits in the circuit board is transmitted to the package through the circuit board, or, as an alternative, is radiated within the internal space of the package. Accordingly, heat can be accumulated within the package that exhibits a low heat radiation effect.
On the other hand, since the conventional electronic device is designed to establish electrical connection between the circuit boards through the lead pins, which are used for externally feeding a signal, when signals are not commonly output externally, the lead pins have to be dummy pins, thereby resulting in a reduction in the number of active pins for externally outputting the signals.
Also, since the electrical wiring between a plurality of circuit boards is used in common to the lead pins, the number of connections to be established between the circuit boards is limited. In the case of the electronic device in which the packaging density of the elements is increased, the amount of wiring for connecting the circuit boards becomes large. Therefore, solving this problem is an important task.
In such an electronic device, bonding of the circuit board onto the package has been performed without adequate care. Therefore, an excessive amount of adhesive on the bonding surface may bulge to the surface of the circuit board, or may drop on the lower circuit board.
Such an excess amount of adhesive can serve to increase the floating electrostatic capacity or cause a dielectric breakdown, or can be a cause of a failure of electric connection by adhering to the bonding portion on the circuit board. Also, when the adhesive adheres to the connection lead for an electrical connection between the circuit boards, an abnormal transmission of the bonding power can occur in the connecting process of the substrate and the connection lead by way of wire bonding to result in a bonding failure. Furthermore, it is possible that the heat radiation effect is lowered because of the adhesive, thus causing distortion, cracking, degradation of the characteristics owing to a difference in thermal expansion coefficients between the adhesive, the substrate and the connection leads and so forth.
On the other hand, it is typical to coat the upper portion of the circuit board installing the electronic circuit with a gel form coating agent for moisture-proofing. In such a case, a coating agent that is in the liquidous state at the initial condition, can bulge up along the side wall inside of the frame because of surface tension thereof to adhere on the horizontal surface of the installation guide for the upper circuit board and thus cause a failure of connection between the upper circuit board and the frame. Also, the gel form coating agent can contain foam and therefore degrade the moisture-proofing effect.
Furthermore, when the coating agent adheres on the connection lead for an electrical connection between the circuit boards, it can cause an electrical connection failure similar to the adhesive agent.